This invention relates generally to the automated testing, optimization and harmonization of the performance measurements of visual displays. Currently, systems are available to automatically test visual displays by providing measurements on display characteristics (for example: luminance, transmission level, contrast ratio, luminance uniformity, chromaticity uniformity, viewing angle dependence, and luminous efficiency) of the visual displays. Current systems may be capable of measuring, gathering and comparing the display characteristics. Some even enhance the value of a certain display characteristics, such as luminance uniformity, to the detriment of other characteristics, such as contrast ratio. They do not, however, automatically provide for the optimization of all display characteristics as may be done by the present invention. The present invention may be used with any standard automated or non-automated system for testing digital flat panel displays. One such automated system for measuring the optical performance of a visual display under test is the method described in U.S. Pat. No. 6,177,955 (which is hereby incorporated by reference in its entirety) and is embodied in the Display Tuning System manufactured by Westar Corporation located in St. Louis, Mo.
One type of visual display that can be tested, optimized and harmonized with the present invention is an active matrix liquid crystal display (AMLCD). AMLCD's are well known in the art, and depend on thin film transistors (TFT's) and capacitors to maintain an isolated charge at each subpixel until the next refresh cycle. They are arranged in a matrix on one of the glass panels between which is sandwiched the liquid crystal material. To address a particular subpixel, a gate voltage is applied to a row, switching on that row's transistors and thereby letting that row's subpixels accept a charge. Voltages (“gray level voltages”) are applied to the columns corresponding to the light transmission level desired at individual subpixel elements at the intersection of the column and row in question. Since the other rows that the column intersects are turned off, only the capacitor at the designated subpixel receives a charge from a particular column.
The voltage potential differential between the front glass panel and a subpixel TFT controls the amount of “untwisting” accomplished by the twisted nematic liquid crystalline material at the subpixel element. This level of untwisting, in turn, determines the amount of light, which the material permits to pass through the front glass panel. By controlling the voltage applied to the subpixels, LCD's can create a gray scale. In one type of LCD monitor the liquid crystals organize into a structure that makes the subpixels transparent in the absence of a voltage differential.
A net voltage potential should not be maintained across the cell gap between the glass plates for an appreciable time or electroplating of the liquid crystalline material will occur, and image retention will result. A variety of driving schemes are known in the field to avoid the said electroplating phenomenon. One way to avoid electroplating is to minimize the voltage potential being maintained across the cell gap by supplying an alternating polarity voltage potential to each subpixel TFT relative to the common voltage of the opposite plate (Vcom).
Knowing the voltage applied to a given subpixel TFT and the common voltage of the opposing plate (Vcom) will not directly determine the actual voltage potential present at each subpixel element. TFT electrical distortion and threshold offsets, among other factors, cause the charge present at a subpixel TFT to differ in an undetermined way from the applied voltage. As there is no way to directly measure this charge present at the TFT, the only way to determine the actual voltage potential of a subpixel element is through indirect means, such as by measuring the resulting level of light transmission.
With respect to the alternating voltage potentials applied to the subpixel TFT's, if the magnitude of the positive and negative potentials at the subpixels relative to Vcom are different the light transmission level will appear to flicker as the panel refreshes. This flickering occurs because the liquid crystal switches from one orientation to the opposite depending on the polarity of the potential, and the magnitude of light transmission is determined by the magnitude of that potential. If the magnitude of the positive potential differs from the magnitude of the negative potential, the light transmission changes as the waveform changes from positive to negative, and vice versa. This “unbalanced” state resulting in flicker increases the likelihood of electroplating since a nonzero voltage potential is effectively maintained across the cell gap. “Harmonizing” an LCD display implies balancing, or correcting, this unbalanced state.
By electrically balancing, or harmonizing, a panel to a high degree of accuracy, the present invention prevents image retention, as described above, and allows for the setting of the optimum, or maximum, voltage potential range, resulting in, among other characteristics, maximum contrast ratio and maximum luminance, or light transmission level. Monotonicity is maintained and flicker is minimized. Through automation, the present invention provides for a time-efficient and highly repeatable method of harmonizing panels by generating voltage setting correction factors and storing them into memory that is incorporated into the display panel. The harmonization optimizes the panel's viewing characteristics irrespective of viewing angle.
End users frequently desire tailored light transmission level versus gray scale voltage curves (“gamma curves”) depending on their applications. While harmonizing as described, the present invention may also provide for the tailored adjustment of the gamma curve through correction factors to fit the user's request.
An exemplary embodiment of the present invention is to a system and method for automatically optimizing the display performance characteristics for a visual display by harmonizing and tailoring the display's voltage settings and storing into memory the voltage level correction factors for each visual display.
The exemplary embodiments herein disclosed are not intended to be exhaustive or to unnecessarily limit the scope of the invention. The exemplary embodiments were chosen and described in order to explain the principles of the preset invention so that others skilled in the art may practice the invention. Having shown and described exemplary embodiments of the present invention, those skilled in the art will realize that many variations and modifications may be made to affect the described invention. Many of those variations and modifications will provide the same result and fall within the spirit of the claimed invention. It is the intention, therefore, to limit the invention only as indicated by the scope of the claims.